The reason for this problem is that the certificate of the installation source is not trusted. You can change the configuration file of pip to solve this problem:
Where is the profile first to change it? Read the source code of pip:
Here is the pip source code

if WINDOWS:
    bin_py = os.path.join(sys.prefix, 'Scripts')
    bin_user = os.path.join(user_site, 'Scripts')
    # buildout uses 'bin' on Windows too?
    if not os.path.exists(bin_py):
        bin_py = os.path.join(sys.prefix, 'bin')
        bin_user = os.path.join(user_site, 'bin')

    config_basename = 'pip.ini'

    legacy_storage_dir = os.path.join(user_dir, 'pip')
    legacy_config_file = os.path.join(
        legacy_storage_dir,
        config_basename,
    )
else:
    bin_py = os.path.join(sys.prefix, 'bin')
    bin_user = os.path.join(user_site, 'bin')

    config_basename = 'pip.conf'

    legacy_storage_dir = os.path.join(user_dir, '.pip')
    legacy_config_file = os.path.join(
        legacy_storage_dir,
        config_basename,
    )
    # Forcing to use /usr/local/bin for standard macOS framework installs
    # Also log to ~/Library/Logs/ for use with the Console.app log viewer
    if sys.platform[:6] == 'darwin' and sys.prefix[:16] == '/System/Library/':
        bin_py = '/usr/local/bin'

Can be found
The path under windows is: [user folder] / pip/pip.ini
Other systems are: [user folder] / pip/pip.conf

I am in the windows system, and then enter user > administrator

No PIP folder found. It doesn't matter. We can create a new folder by ourselves named pip

And create the pip.ini file under the PIP folder

Open with notepad and enter the following configuration information

[global]  
index-url = http://pypi.douban.com/simple
trusted-host = pypi.douban.com
disable-pip-version-check = true
timeout = 120  

[list]
format = columns

Index URL is the Douban source set here
Trusted host means to trust this address (which eliminates ssl validation)
Disable PIP version check = true set not to check version

format = columns here is the output style when using the pip list command. Enter pip config list
Effect:

Seeing this effect, it proves that the configuration file has taken effect.